A very frequently encountered embodiment of such a bearing is found between the back and mid constituents of the boom of self-propelled excavators, on which one boom cylinder acts on each side, as a third component. In this case the lower end of the back boom is mounted in a drag link on the frame of the revolving superstructure. The upper end of the upward-pointing back boom takes the form of an upward-opening fork, in which the rear area of the mid boom is accepted in said bearing, at a certain distance in front of the rear end of the mid boom, seen in the direction of movement of the self-propelled excavator, both parts being connected to each other by means of a pin.
To facilitate the swivel movement of the back boom, one end of the aforementioned hydraulic boom cylinder is mounted on the outer end of the bearing pin and the other end is articulated to a knuckle on the frame of the revolving superstructure at a certain distance from the drag link of the back boom. Both connections may be made either by cylindrical bearings or drag bearings.
Purely for the sake of completeness, it should be mentioned in respect of the design of the boom in this area that a hydraulic adjustment cylinder, the lower end of which is mounted on a bracket on a fixed pivot on the lower part of the back boom, acts on the rear end of the mid boom to raise and lower it. The front end of the mid boom and the downstream components, seen in the direction of movement of the self-propelled excavator, need not be considered further in this context as they are located too far away from the bearing and have no inherent connection with it.
From its center outwards, in cross-section, the bearing itself is designed so that the two bearing points of the mid boom are in the middle of the pin, into the borings of which penetrated by the pin guide bushes are pressed. The back boom envelopes the mid boom from below and outside, and accepts the pin close to the latter's outer ends in two plain bearings, into which guide bushes are also inserted, in the vicinity of its upper edge. The two ends of the pin protruding from the plain bearings are encompassed by the bosses of the boom cylinder, in which the aforementioned cylindrical bearings or drag bearings are located. End discs or rings attached externally prevent their axial displacement on the pin.
This solution has several disadvantages, namely that the connection between the three components, the back boom, mid boom and boom cylinder by means of the pin must accept the forces from all the components on its own in this embodiment, as a result of which it must have a correspondingly large diameter. However, a large diameter of the bearing point for the bosses of the boom cylinder means that the stroke of the boom cylinder and thus the maximum reach of the boom of the self-propelled excavator would be restricted. As this would be too serious of a disadvantage, the outer ends of the pin are turned down to a smaller diameter, entailing, however, a new disadvantage in the form of higher manufacturing costs. A further shortcoming arises from the use of cylindrical bearings for the aforementioned upper and lower mountings of the boom cylinder, because removal of the boom cylinders in this embodiment is then only possible if both connections are released and the boom cylinders are drawn out evenly to the side, to prevent tilting. Accordingly, analogous problems also arise on assembly. Although this assembly/removal problem is alleviated if drag bearings are used instead of cylindrical bearings, two new disadvantages emerge. Firstly, the drag bearings have a greater diameter than cylindrical bearings and thus restrict the reach of the boom, as aforementioned, and entail increased costs. Finally, a significant shortcoming of such bearings is that the back and mid boom fall apart when the pin is taken out when they are removed.